Since lithium ion secondary batteries have a high electromotive force and high energy density, they are employed as main power sources for mobile communication devices and portable electronic devices. A typical lithium ion secondary battery comprises a positive electrode composed of a lithium composite oxide, a negative electrode composed of a material capable of absorbing and desorbing lithium ions, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.
Separators serve to electronically insulate the positive electrode and the negative electrode from each other as well as to retain a non-aqueous electrolyte. A separator is typically produced by molding a polyolefin resin or the like into a sheet form. Separators usually deform at a temperature of 120° C. to 160° C. For this reason, when a sharp projection such as a nail penetrates a separator (e.g., as in the nail penetration test), the separator deforms around the projection due to heat that is instantly generated by short-circuiting, thereby enlarging the short-circuited area. As a result, the battery might reach an overheated state.
In order to cope with the problem, it has been proposed to adhere a film containing an inorganic oxide such as alumina and a binder on the surface of either positive electrode or negative electrode (see Japanese Laid-Open Patent Publication No. Hei 7-220759). However, when a film is adhered to the electrode surface, discharge characteristic of the battery, namely, discharge characteristic in a low temperature environment or during large current discharge might be deteriorated significantly.
Also, a technique is proposed to enhance the shut-down effect of a film in the event of internal short-circuit by forming the film composed of a resin material having a high bulk density on an electrode (see Japanese Laid-Open Patent Publication No. Hei 11-144706). The effect of inhibiting ion migration obtained by allowing the resin forming a separator or film to soften and closing the pore structure is called shut-down effect.
In order to allow such a film to exhibit its shut-down effect in the event of internal short-circuit, it is necessary to set the glass transition temperature of the resin material at a low level. During the nail penetration test, however, the short-circuited area could locally have a temperature of over several hundred degrees although the temperature may vary depending on the test conditions. Accordingly, there is a possibility that the resin having a low glass transition temperature might be excessively softened or burned out, and the short-circuited area might be enlarged.
Proposals are also made from the viewpoint of preventing an internal short-circuit due to the asperity of electrode surface, one of which is a technique to form a film composed of an inorganic oxide filler such as alumina and a water soluble polymer on an electrode (see Japanese Laid-Open Patent Publication No. Hei 9-147916). The use of a film composed of an inorganic oxide filler having superior thermal resistance and a water soluble polymer prevents the film from deforming in the event of internal short-circuit.
However, polymers often swell by absorbing a liquid component for dispersing the raw material of the film during the formation of the film or absorbing an electrolyte during charge/discharge. If the film swells, the number of ion migration paths is decreased, resulting in low ion conductivity between the electrode plates, which makes it difficult to maintain discharge characteristic of the battery. Therefore, unless something is done to prevent the swelling of the film, even if improved safety against short-circuit is achieved, it is difficult to keep discharge characteristic of the battery.
Meanwhile, from the viewpoint of preventing dendrites, a technique is proposed in which a separator composed of a polymer layer having a porous structure and a ceramic composite layer containing inorganic particles is used (see Japanese Laid-Open Patent Publication No. 2001-319634). Further, in the event of lacking an electrolyte between the positive and negative electrodes due to the swelling of the electrodes, from the viewpoint of supplying an electrolyte to between the electrodes, a technique is proposed in which an electrolyte-retaining layer including inorganic particles dispersed therein is formed on the side of a separator in contact with a negative electrode (Japanese Laid-Open Patent Publication No. 2002-8730).
The foregoing improvement techniques (e.g., Japanese Laid-Open Patent Publications Nos. 2001-319634 and 2002-8730) are intended to prevent dendrites or to improve high rate discharge characteristic, and they do not deal with safety against internal short-circuit and safety at the time of nail penetration test. The ceramic composite layer is a part of the separator and the electrolyte-retaining layer is integrated with the separator. Accordingly, in the event of internal short-circuit, the ceramic composite layer and the electrolyte-retaining layer will also deform due to heat generated from the short circuit reaction.